Tammar wallaby spermatozoa undergo maturation during transit through the epididymis. This maturation differs from that seen in eutherian mammals because in addition to biochemical and functional maturation there are also major changes in morphology, in particular formation of the condensed acrosome and reorientation of the sperm head and tail. Of spermatozoa released from the testes, 83% had a large immature acrosome. By the time spermatozoa reached the proximal cauda epididymis 100% of sperm had condensed acrosomes. Similarly 86% of testicular spermatozoa had immature thumb tack or T shape head-tail orientation while only 2% retained this immature morphology in the corpus epididymis. This maturation is very similar to that reported for the common brush tail possum, Trichosurus vulpecula. However, morphological maturation occurred earlier in epididymal transit in the tammar wallaby. By the time spermatozoa had reached the proximal cauda epididymis no spermatozoa had an immature acrosome and thumbtack orientation. Associated with acrosomal maturation was an increase in acrosomal thiols and the formation of disulphides which presumably account for the unusual stability of the wallaby sperm acrosome. The development of motility and progressive motility of tammar wallaby spermatozoa is similar to that of other marsupials and eutherian mammals. Spermatozoa are immotile in the testes and the percentage of motile spermatozoa and the strength of their motility increases during epididymal transit. During passage through the caput and corpus epididymis, spermatozoa first became weakly motile in the proximal caput and then increasingly progressively motile through the corpus epididymis. Tammar wallaby spermatozoa collected from the proximal cauda epididymis had motility not different from ejaculated spermatozoa. Ultrastructural studies indicated that acrosomal condensation involved a complex infolding of the immature acrosome. At spermiation the acrosome of tammar wallaby spermatozoa was a relatively large flat or concave disc which projected laterally and anteriorly beyond the limits of the nucleus. During transit of the epididymal caput and proximal corpus the lateral projections folded inwards to form a cup like structure the sides of which eventually met and fused. The cavity produced by this fusion was lost as the acrosome condensed to its mature form as a small button-like structure contained within the depression on the anterior end of the nucleus. During this process the dorsal surface of the immature acrosome and its outer acrosomal membrane and overlying plasma membrane were engulfed into the acrosomal matrix. This means that the dorsal surface of the acrosomal region of the testicular tammar wallaby sperm head is a transient structure. The dorsal acrosomal surface of the mature spermatozoon appears ultrastructurally to be the relocated ventral surface of the acrosomal projections which previously extended out beyond the acrosomal depression on the dorsal surface of the nucleus of the immature spermatozoon.